Hot dip aluminum coating process



United States Patent HOT DIP ALUMINUM COATING PROCESS Webster Hodge, Columbus; Ohio, assignor, by mesne assignments, to Surface Combustion Corporation, Toledo, Ohio, a corporation of Ohio No Drawing. Application August 19, 1953, Serial No. 375,299

9 Claims. (Cl. 117-51) The invention relates to a process of coating ferrous metals, and in particular to a process of coating ferrous metals by dipping in molten aluminum.

Coatings that have been applied to ferrous metals by dipping in molten aluminum have been found to provide excellent resistance to corrosion. However, it has been found to be extremely difficult in commercial practice to cause the molten aluminum to adhere throughout the surface of the ferrous metal during the dipping operation so as to provide the clipped article with a continuous, complete coating.

U. S. Patent No. 2,408,623 discloses that iron or steel may be provided with a satisfactory coating by dipping in a bath of molten aluminum if the article to be dipped is cleaned and then, while still wet with water, is immersed in the bath of molten aluminum. That patent discloses that it is essential that all of the surface to be coated with aluminum be wet with water at the moment when the article is immersed in the bath of molten aluminum.

The process disclosed in this patent, which requires that the ferrous metal article be wet with water over its entire surface at the time when it is immersed in the molten aluminum, is satisfactory for applying an aluminum coating to wire, but is not satisfactory when the article to be immersed in the molten aluminum is large or is of a complicated shape, because the surface of such an article would carry so much water as to cause a virtual explosion upon immersion of the article in the molten aluminum. Moreover, it is usually inconvenient in commercial practice to keep an article wet with water-over its entire surface from the time when the article is cleaned until the time when the article is dipped in molten aluminum.

The principal object of the invention is to provide a process in which ferrous metal articles, after being cleaned, may remain dry while in contact with the atmosphere for a reasonable time, before immersion of the articles in a bath of molten aluminum. More specific objects and advantages are apparent from the description, which discloses and illustrates but is not intended to limit the invention.

The present process, in which a ferrous metal is provided with a hot-dipped coating consisting essentially of aluminum, comprises the novel steps of preparing the cleaned surface of the ferrous metal to receive such coating, by providing on such surface a coating of an aqueous solution of an akaline salt of an oxyacid of phosphorus with an alkali metal, having'a concentration from .05 to 2 per cent, and drying.

PREPARATORY CLEANING OF FERROUS METAL In the present process, which is applicable to all ferrous metals, including all types of iron or steel, the preparatory cleaning operation is carried out in accordance ice The removal of grease or oil may be carried out by heating to a temperature of at least 1000 F., and preferably 1200 F. At these temperatures, the oil or grease burns oif rapidly. In place of such a burning step, it is permissible to use any other conventional method, such as treatment with vapors of trichloroethylene, or treatment with other organic liquids which are commonly used for this purpose.

The second step of the preparatory cleaning operation consists in the removal of oxides from the surface of the ferrous metal. This step also may be carried out according to conventional practice. The most common way of removing oxides from a ferrous metal in commercial practice consists in pickling in a sulfuric acid solution, and the process of the present invention is particularly advantageous when used in conjunction with pickling in sulfuric acid solution. In accordance with usual practice, the concentration of the sulfuric acid solution may be about ten per cent, and the pickling in such a solution, at a temperature of -180 F., may be carried out by immersion for a period of two to eighteen minutes, depending upon the amount of oxide on the surface.

Inhibitors may be used in the pickling solution in order to control acid corrosion and hydrogen embrittlement, in accordance with usual practice. Any of the various commercial inhibitors may be employed, but the amount of the inhibitor should not be so great that the surface of the ferrous metal is not etched by the pickling solution. it has been found that slight etching of the surface of the ferrous metal during the pickling operation is desirable because it improves the adherency of the final aluminum coating. As in other pickling operations, it is possible to use a solution of hydrochloric acid or chromic acid instead of sulfuric acid. Moreover, the pickling operation may be replaced by a sand blasting operation or by other known methods such as anodic or cathodic cleaning in hot alkaline baths.

The third step of the preparatory cleaning operation consists in rinsing with hot water. This step is necessary if an acid pickling step or other solution treatment has been used to remove oxides. If the oxides have been removed by a sand blasting operation rather than by solution treatment, it is possible to omit the step of rinsing in hot water provided that the sand or other abrasive is clean and the sand blasting operation is followed within 10 minutes by the step of treatment with a solution of an alkaline salt of an oxyacid of phosphorus.

TREATMENT WITH SOLUTION OF ALKALINE SALT OF OXYACID OF PHOSPHORUS This step should follow closely after the preparatory cleaning operation, without permitting the surface of the cleaned ferrous metal to be exposed to the atmosphere for more than 10 minutes at ordinary temperatures after the hot water rinse or sand blasting operation. The surface of the cleaned ferrous metal appears to be extremely active, and if it is permitted to remain in contact with the atmosphere for an appreciable time after a cleaning operation, a change takes place that prevents the molten aluminum from adhering properly to the surface of the ferrous metal during the subsequent hot dipping operation. After such change, the ferrous metal is no longer considered to be clean. The purpose of the treatment with the solution is to condition the surface of the clean ferrous metal so that the surface then can be permitted to dry and remain in contact with the atmosphere for a reasonable period of time prior to the hot dipping operation, as required in ordinary commercial production.

The treatment with the salt solution may be carried out at ordinary temperatures. However, the best results are obtained by employing a vigorously boiling solution of the salt. The treatment is completed by mere immersion of the ferrous metal in the solution.

The solution employed in this treatment may be an aqueous solution of any alkaline salt of an oxyacid of phosphorus with an alkali metal. The most common salts of this type are trisodium phosphate and dibasic sodium phosphate. Monobasic sodium phosphate is not an alkaline salt and cannot be employed in this step. Another salt that can be employed is sodium pyrophosphate. Salts of other oxyacids of phosphorus generally are oxidized or hydrolyzed to form phosphates in dilute aqueous solution, so that for practical purposes the important alkaline sodium salts of oxyacids of phosphorus are those which have been mentioned. Potassium salts can be used instead of sodium salts, but are less desirable because they are more expensive.

The concentration of the salt in this solution may range from .05 to 2 per cent. In the case of trisodium phosphate, a solution having a concentration of about 0.2 per cent gives satisfactory results even when a period of sixty minutes elapses between the time when the solu tion is dried on the ferrous metal and the time when the ferrous metal is dipped in the molten aluminum. However. it is good practice to use a concentration of about 0.38 per cent in the case of a solution of trisodium phosphate. When a 0.38 per cent solution of this salt is employed, an excellent aluminum coating may be applied by hot dipping the ferrous metal as long as two hours after the salt solution has been dried on the ferrous metal. Other salts that are slightly less effective may be employed in larger concentrations. For example, it may be desirable to use a concentration in the case of dibasic sodium phosphate that is approximately twice the concentration employed in the case of trisodium phosphate.

The salt solution employed should have a pH of at least 8.5. An alkaline salt of an oxyacid of phosphorus with an alkali metal gives such a pH. For example, a 0.19 per cent solution of trisodium phosphate has a pH of about 12, and a 1.5 per cent solution of the same salt has a pH only very slightly greater. A 0.38 per cent solution of sodium pyrophosphate has a pH from 10.5 to ll, a 0.75 per cent solution of dibasic sodium phosphate has a pll of 9 to 9.4. The solution may be prepared from tap water, which ordinarily has a slight alkalinity that varies from day to day. In fact, the variation in tap water from day to day usually has more effect on the pH of the solution than changes in the concentration of the salt employed. So long as the pH of the solution is at least 8.5, dififerences in pH have no appreciable effect on the results obtained.

After the treatment with the solution, the ferrous metal may be permitted to dry by mere exposure to the atmosphere, or may be dried by forced circulation of warm air. A convenient way of drying consists in the use of an air tunnel of the conventional type. The ternperature of the air used may be as high as 250 to 350 F. At such temperatures, all moisture is quickly removed from the ferrous metal.

APPLICATION OF ALUMlNUlt l COATING The hot dipping operation for applying the aluminum coating may be carried out in accordance with usual commercial practice. The molten metal in the oath consists essentially of aluminum, although other metals such as zinc or tin may be included in the bath. If other metals are included, the molten metal should consist of at least 90 per cent aluminum by weight.

The present process makes it possible to eliminate the use of fluxes with their attendant disadvantages.

The temperature of the bath of molten metal may be any temperature above the melting point. However, it is preferable that the temperature of the molten metal be about l350 F. in order to facilitate the draining of the excess molten metal upon removal of the ferrous metal article from the bath, particularly if the article is of complicated shape. At this high temperature, the metal of the coating may absorb some iron from the underlying surface so that the resulting coating consists to some extent of an iron-aluminum alloy. In addition, some iron may dissolve in the bath. In one operation at this temperature, the iron content of the molten metal rose from 0.72 per cent to 1.02 per cent over a period of two months during which the total amount of steel dipped in the bath was 500 pounds. During the same period, the silicon content of the bath increased from 0.4 per cent to 1.72 per cent, by reason of the attack of the aluminum on the refractories used to hold the bath. Such accumulation of iron and silicon in the bath was not detrimental to the aluminum coating produced by hot dipping.

in order to retard the dissolving of iron in the molten metal, the bath may be kept at a temperature as low as 1250 F., if a thick coating of aluminum is desired or if satisfactory arrangements can be made to expedite the draining of the excess metal from the dipped articles at such temperatures.

The dipping apparatus, in accordance with accepted practice, should be adapted to immerse the ferrous metal article in such a manner as to prevent the entrapment of air.

The period of time during which the ferrous metal article is immersed in the bath of molten metal depends upon the shape of the article and the nature of the coating desired. in accordance with usual practice, the duration of the immersion of the article in the molten metal may vary from a few seconds to as much as ten minutes in the case of articles of complicated shape.

Even though 99 per cent or more of the metal in the bath consists of aluminum, the bath may contain other ingredients. For example, the bath may contain as much as 5 per cent of silicon. Ordinary commercial aluminum, containing up to 1 per cent of impurities, may be employed, or fractional percentages of modifying substances may be included in the bath in order to modify its properties in accordance with known practice.

Known expedients may be employed to prevent or minimize the formation of an oxide film on the surface of the bath of molten metal. One such expedient consists in the employment of a protective atmosphere or a layer of a flux above the bath of molten metal. Another expedient is the addition to the bath of 0.2 per cent of its weight of metallic sodium. As the result of this addition a saturated solution of sodium is maintained in the bath. The sodium lowers the surface tension of the molten metal, in addition to decreasing the thickness of the oxide film on the surface of the bath. As a result, an article of ferrous metal is not enveloped, at the moment when it is immersed in the molten metal bath, by air which would oxidize the ferrous metal before it became coated by the molten aluminum. The sodium acts as a Wetting agent.

Another expedient that may be used to insure obtaining satisfactory coatings on a large article of ferrous metal consists in coating with wax the end of the article that is immersed in the molten metal last, so that the wax burns on the surface of the molten metal, providing a non-oxidizing atmosphere.

The phrase coating consisting essentially of aluminum, as used herein, means a coating containing per cent or more of aluminum. Per cent or parts, as used herein, mean per cent or parts by weight, unless otherwise qualified.

The practice of the present invention is illustrated by the following specific examples:

Example 1 Small panels of number 1010 mild steel were immersed for two minutes at about F. in a solution prepared by mixing 1 part by volume of water with l part by volume of hydrochloric acid having a specific gravity of 1.18. The panels then were rinsed in warm water; then half of the panels were immersed for a few seconds in a warm solution of a salt; and all of the panels were hung up to dry for thirty minutes and then were immersed for two minutes at a temperature of 1350 F. in a bath that contained 4 per cent of silicon, 0.2 per cent of chromium, 0.2 per cent molybdenum, .03 per cent of titanium and asmall amount of sodium (between .003 per cent and .05 per cent). In the case of some of the panels, the salt solution employed consisted of /2 ounce of trisodium phosphate dissolved in a gallon of Water; in the case of some of the other panels, the salt solution consisted of 1 ounce of dibasic sodium phosphate dissolved in a gallon of water; and in the case of some of the other panels the salt solution consisted of %ounce of sodium pyrophosphate dissolved in a gallon of water.

The entire procedure then was repeated, except that each of the salt solutions was boiling when the panels were dipped therein, and the panels were hung up to dry for two hours instead of thirty minutes before dipping in the molten metal.

Each of the panels that had been dipped in a salt solution was found to have received an excellent uniform coating of aluminum, whereas nearly all of the panels that had not been dipped in a salt solution were found to have defective aluminum coatings.

Example 2 The procedure of Example 1 was repeated, except that the pickling solution consisted of a solution at 175 F. that had been prepared by mixing 1 volume of sulfuric acid having a specific gravity of 1.84 with 9 volumes of water; the salt solutions were boiling; and the panels were hung up to dry for 1 /2 hours. Among the panels so prepared, all of the panels that had been dipped in a salt solution acquired excellent aluminum coatings, whereas all of the panels that had not been dipped in a salt solution were found to have received defective aluminum coatings.

Example 3 Sample panels of 16-gage cold-rolled steel were heated to 1200 F. to burn 01f all grease, immersed for four minutes at 175 F. in the sulfuric acid solution used in Example 2, rinsed in hot water, dipped in a boiling trisodium phosphate solution, held in contact with air for a predetermined period of time, and then immersed for two minutes at 1350 F. in a molten metal which had the same composition as the molten metal used in Example 1 except that the silicon was omitted and sodium was present in excess of the amount that would dissolve.

In one case, in which the concentration of the salt solution was 0.125 ounce per gallon and the holding time in contact with air was fifteen minutes, the panel received an excellent coating. However, a panel dipped in this salt solution and held for thirty minutes in contact with air was found to have received an incomplete coating of aluminum. Also, a panel that was merely rinsed in hot water, without being immersed in a salt solution, was found to receive an incomplete coating of aluminum after being held in contact with air for fifteen minutes.

In these tests, panels were dipped also in trisodium phosphate solutions having concentrations of 0.25, 0.5, 1.0, 2.0 and 3.0 ounces per gallon, respectively, and were exposed to the atmosphere for periods ranging from one to sixty minutes. All of the resulting panels were found to have excellent coatings of aluminum, with the exception of the panels that had been dipped in-the solution having a concentration of 3.0 ounces per gallon. All the panels that had been dipped in the latter solution were found to have very rough aluminum coatings.

Having described the invention, I claim:

1. In a process of providing a ferrous metal with a hot-dip coating consisting essentially of aluminum, the steps of preparing the cleaned surface of the ferrous metal to receive such coating, by providing on such surface a coating of an aqueous solution of an alkaline salt of an oxyacid of phosphorus with an alkali metal, having a concentration from .05 to 2 per cent, and drying.

2. A process as claimed in claim 1 wherein the cleaned ferrous metal is dipped in the coating solution while the coating solution is boiling.

3. A process as claimed in claim 1 wherein the salt is trisodium phosphate.

4. A process as claimed in claim 1 wherein the ferrous metal is a mild steel.

5. A process of coating a ferrous metal that comprises cleaning the ferrous metal, providing on the cleaned ferrous metal a coating of an aqueous solution of an alkaline salt of an oxyacid of phosphorus with an alkali metal, having a concentration from .05 to' 2 per cent, drying, and dipping the ferrous metal, having the resulting dried coating, in a bath of molten metal consisting essentially of aluminum.

6. A process as claimed in claim 5 wherein the cleaned ferrous metal is dipped in the coating solution while the coating solution is boiling.

7. A process as claimed in claim 6 wherein the salt is trisodium phosphate. 7

8. A process as claimed in claim 5 wherein the cleaning step comprises pickling in an acid solution and then rinsing with water.

9. A process as claimed in claim 8 wherein the acid solution is a sulfuric acid solution.

References Cited in the file of this patent UNITED STATES PATENTS 213,319 Brown et a1 Mar. 18, 1879 214,360 Brown Apr. 15, 1879 1,804,991 Johnson May 12, 1931 

1. IN A PROCESS OF PROVIDING A FERROUS METAL WITH A HOT-DIP COATING CONSISTING ESSENTIALLY OF ALUMINUM, THE STEPS FO PREPARING THE CLEANED SURFACE OF THE FERROUS METAL TO RECEIVE SUCH COATING, BY PROVIDING ON SUCH SURFACE A COATING OF AN AQUEOUS SOLUTION OF AN ALKALINE SALT OF AN OXYACID OF PHOSPHORUS WITH AN ALKALI METAL, HAVING A CONCENTRATION FROM .05 TO 2 PER CENT, AND DRYING. 